Heretofore, small and lightweight imaging devices which include a solid-state image sensor, such as a CCD, CMOS, or the like, and a single focus imaging lens have been known. Such imaging devices include, for example, digital cameras, surveillance cameras, in-vehicle cameras, image reading cameras for deficiency detection, individual identification, and the like. As imaging lenses that form an optical image on a light receiving surface of such solid-state image sensors, bright, low distortion, and high resolution imaging lenses while still having a simple structure with a small number of lenses and an appropriate back focus for inserting an optical element, such as a filter or the like, have been known (refer to Japanese Unexamined Patent Publication No. 10 (1998)-300906, Japanese Unexamined Patent Publication No. 2002-244031, U.S. Pat. No. 6,762,890, Japanese Patent No. 4416411, U.S. Pat. No. 7,312,930, Japanese Unexamined Patent Publication No. 2006-317800, and U.S. Pat. No. 7,142,372).
An imaging lens used for recording a color image, in particular, is preferably aberration-corrected such that, for example, each optical image with respect to each of the wavelength ranges of red (R), green (G), and blue (B) substantially corresponds to each other on the light receiving surface and such that the contrast of each optical image formed on the light receiving surface becomes equal to each other. This means that various types of aberrations need to be corrected satisfactorily in a wide wavelength region. More specifically, for example, an optical image of each color of RGB may be formed on the light receiving surface with high resolution by reducing the deviation of spherical aberration and longitudinal and lateral chromatic aberrations, and matching the image location (image plane) from the center of the optical image to periphery with respect to each color of RGB.
Further, in order to downsize the device, it is necessary to reduce the overall optical length (distance from the first surface of the imaging lens to the image plane of the imaging lens), as well as downsizing the imaging lens itself.
In a case where the angle of view of the imaging lens is broadened, it is necessary to shorten the focal length, but a significant focal length reduction may result in a reduced back focus. Therefore, in order to insert, for example, a low-pass filter, the imaging lens needs to be designed to have a large back focus in comparison with the focal length.
Further, in a camera that uses a solid-state image sensor, if the distance from the exit pupil of the imaging lens to the image plane is extremely short, the incident angle of an off-axis light beam on the light receiving surface becomes large and uneven illuminance is increased due to the effect of shading. Therefore, a lens system having telecentricity in which the image plane is sufficiently away from the exit pupil is used in an imaging lens used for forming an optical image on the light receiving surface of such solid-state image sensor.
As a means for realizing the telecentricity, use of a retrofocus type lens system formed of a lens system having a negative refractive power and a lens group having a positive refractive power arranged in order from the object side is known.
The retrofucus type lens system allows a long back focus in comparison with the focal length and the angle between the principal ray of an off-axis light beam and the optical axis may be reduced. This may provide a sufficient distance from the exit pupil of an imaging lens to the image plane of the imaging lens, so that it can be said that the employment of the retrofocus type lens system in an imaging lens that requires telecentricity is appropriate.